All non-destructive testing and evaluation (NDT&E) techniques utilize some sort of probing radiation and characterize the target by measuring the scattered, reflected, or attenuated radiation. X-ray tomography and CAT scan image a target by measuring its X-ray attenuation. Sonogram maps a target by scanning the reflection of ultrasonic radiation from different layers of the target. Nuclear magnetic resonance (NMR) produces detailed pictures of a target with respect to the relaxation time of its molecules' magnetic moment. Neutron scattering is used to detect content of some atomic elements in the material. Since different radiation interacts with the target through different physical mechanism, the image thereby obtained represents only the image of the target with respect to the particular physical property that is responsible for such interaction. Among these existing NDT&E techniques, there is a missing gap of imaging that provides dielectric data in the RF to microwave regime. Since there is a vast data resource on dielectric permittivity of various materials and its temperature and pressure dependencies, dielectric imaging offers a valuable means to evaluate material, temperature, and pressure profiles of a composite object or a complex system. Dielectric permittivity of new materials can easily be measured by a network analyzer, which is commercially available, and added to the data bank for image evaluation.
The present invention relates to a three-dimensional quantitative dielectric image technology by microwave inverse scattering. There are many advantages of using the present invention over other conventional techniques. The present invention offers three-dimensional dual images of real and imaginary parts of dielectric permittivity while X-ray gives only a two-dimensional picture of attenuating power along the radiation path; though CAT scan by X-ray provides three-dimensional images of the targets' density in terms of X-ray absorption per unit volume, each three-dimensional image requires a large number of X-ray exposures. A neutron scattering device requires a radioactive source, whereas the present invention employs low level non-ionizing radiation. Although a microwave imaging system may not provide as good an image resolution as that of NMR, it provides a three-dimensional profile of dielectric permittivity, which is more sensitive for differentiating materials. Another advantage of the present invention is its transportability and low cost, which is believed to be much lower than that of either a NMR or a CAT scan system. The operating cost of the present invention is also orders of magnitude lower than that of NMR or CAT systems. As to sonogram, it is essentially an echogram, in the sense that it detects only discontinuities in the elastic properties of the target. In many applications, imaging in accordance with the present invention may be complementary to sonograms, because the present invention provides a dielectric image whereas sonogram yields an elastic image. Microwave systems do have limitations. Such systems cannot penetrate materials with high electric conductivity; for a metallic object, it can only image its surface shape or, in some cases, the conductivity profile of its surface.
The conventional concept of imaging by microwave scattering is quite similar to the way one perceives an object by seeing. Light waves are scattered from an object and the scattered waves are then received by nerve endings in the retina of the eyes. The detected signals are relayed to the brain, which then processes the signals and constructs an image, resulting in the way one perceives the object. One cannot see the interior of an object because light dissipates rapidly in most objects and no light is scattered from the interior of the object. Since microwave penetrates rather deeply through most dielectric materials, it can be scattered by the interior of most dielectric objects and propagate to the outside. In accordance with the present invention, data from the scattering measurement may then be processed to construct a three-dimensional image of the object. Even for objects that are highly microwave absorbing, such as a biological target, it has been demonstrated from conventional techniques that a specific absorption rate (SAR) in the order of microwatt per gram to tens of microwatt per gram yields a scattered signal that is sufficient for dielectric image reconstruction.
Some of the above-mentioned imaging techniques produce quantitative images, but most give only qualitative images. The difference between a qualitative image and a quantitative image is analogous to that between "seeing" and "measuring". While "seeing" an object provides useful information, it is more desirable to be able to "measure" the object so that the result can be compared to known experimental data. This is particularly useful for medical diagnosis for the purpose of detecting and discriminating abnormal from normal tissues in advance of biopsy. There is a rich data resource concerning the dielectric permittivity of biological tissues. A three-dimensional profile of dielectric permittivity of a part of a patient's body will furnish a rich resource of diagnostic information. Dielectric dependence on other physical quantities, such as temperature and pressure, also may provide a non-invasive means for measuring these quantities.
Though biological subjects are highly absorbing to microwaves, it has been demonstrated that the scattered microwave provides enough signal for dielectric image reconstruction. Indeed, a two dimensional microwave dielectric image of a canine kidney immersed in a cylindrical water tank of about 30 inches diameter has been produced in the past. L. E. Larsen, et al., "Microwave Imaging Systems for Medical Diagnostic Applications", 6th Annual Conference IEES Engr. in Medicine and Biology Soc., Los Angeles, September 1984. The results demonstrated that penetration depth was not a problem.
In addition to medical applications there is also a need for improved imaging in the non-destructive testing and evaluation of structures such as buildings and bridges and in measuring multiphase flows in power plants. A three dimensional profile of the dielectric permittivity of such objects can provide useful data that is not available with conventional techniques.
The ubiquitous presence of multiphase flows in vital industrial areas of contemporary society, such as conventional and nuclear power generation, is well known. Just as well known is the substantial problems that such flows pose to the design engineer in view of the rudimentary degree with which their behavior is currently understood. Of particular concern is the behavior of multiphase flows in unsteady situations such as may arise in operational transients or, more dangerously, under accident conditions. In many cases, it is difficult to predict even such basic parameters as pressure drop, mean phase content (e.g., void fraction), global heat transfer coefficients, and so forth. In such cases measurement is the only possible approach and a large number of techniques have been devised, tested, and developed for this purpose. The difficulty of the task is underscored by the very large number of papers devoted to this aspect alone of multiphase flow research. But even experiments cannot fully simulate such situations as very rapid transients or large-scale systems, for which the development of theoretical approaches is imperative. However, any modelling effort of practical engineering significance can only be achieved sacrificing the overwhelming amount of information that the accurate description of such flows would require. The averaged equations used for this purpose are therefore, by their very nature, incomplete and do not give rise to a mathematically closed problem.
Microwave imaging addresses simultaneously some of the most important and basic aspects of multiphase flow experimentation, the measurement of void fraction, of the interfacial area, and of the flow topography. The application of the present invention to multiphase flows from its non-intrusiveness, real-time capability, ease of application, and reliability. The present invention provides quantitative dielectric imaging based on inverse scattering of microwaves from the multiphase mixture. In this sense it represents an approach intermediate between the low-frequency impedance gauges and the high-energy radiation scattering techniques that have both been in use for some time. Contrary to these techniques, however, the use of microwave frequencies enables the information contained in the wave-vector and frequency spaces to be fully exploited. The final result is a much more detailed measurement, to a large extent free of the many problems and ambiguities that plague conventional approaches.
A number of experimental techniques rely on the interaction between the multiphase flow and electromagnetic radiation. At the low-frequency end, impedance gauges that operate at kHz or MHz frequencies are limited in their spatial resolution by the "effectively dc" field that they use. At the other extremum of X and gamma rays, the wavelength is so short that no information from wave-number space can be extracted from the signal. The need for the use of microwave frequencies, therefore, becomes very attractive in analyzing multiphase flows.
Accordingly, there is a need for a three dimensional microwave imaging system that can be used in medical application and other non-destructive testing and evaluation uses such as the imaging of structures and multiphase flows, and interrogation of luggages.